Adjustable Transmission Phase Shift Mask

ABSTRACT

A phase shift mask having transmission properties that are dependent at least in part on an intensity of an incident light beam. The phase shift mask has a mask substrate that is substantially transparent to the incident light beam. A first phase shift layer is disposed on the mask substrate. The first phase shift layer has a refractive index that is nonlinear with the intensity of the incident light beam. The refractive index of the first phase shift layer changes with the intensity of the incident light beam on the phase shift mask. By using a first phase shift layer on the phase shift mask that has a refractive index that is non linear with the intensity of the incident light beam, properties of a light beam transmitted through the first phase shift layer, such as interference patterns in the transmitted light beam, can be adjusted by adjusting the intensity of the incident light beam. Thus, in this manner there is provided an additional tool by which the exposure patterns produced by the phase shift mask can be adjusted. In other words, the transmission properties of the phase shift mask are adjustable with the intensity of the incident light beam.

FIELD

This invention relates to the field of integrated circuit fabricationand, in particular, to improved photolithography masks for use infabricating integrated circuit devices and methods for making the masks.

BACKGROUND

As integrated circuits continue to shrink in size, the processes bywhich they are formed are increasingly limited by fundamental physicallaws. For example, in forming structures that are less than about aquarter micron in length, or in other words less than about 250nanometers long, such as gate structures in a metal oxide semiconductordevice, the ability of the radiation used to pattern the structureduring the photolithography process is seriously challenged.Photolithography processes typically use ultraviolet radiation with awavelength of about 248 nanometers to expose the photoresist used topattern the structures. Unfortunately, a beam of light beam with awavelength of 248 nanometers has difficulty in resolving the closelyspaced features in a masking pattern that is not appreciably greaterthan the wavelength of the light beam. Thus, the processes used to formintegrated circuits must necessarily change as even smaller devicefeatures, such as 100 nanometer gate lengths, are desired.

One method of forming devices with smaller features is to useelectromagnetic radiation with smaller wavelengths during thephotolithography process. For example, electromagnetic radiation with awavelength of 193 nanometers provides the ability to pattern featuresthat are about twenty percent smaller than those patterned withelectromagnetic radiation having a wavelength of 248 nanometers.However, moving to steppers and other exposure tools that utilize 193nanometer technology is still insufficient, of itself, to produce 100nanometer features. Radiation with even shorter wavelengths, such as 157nanometers, presents serious cost considerations and other technicalchallenges. Thus, other improvements to the photolithography process arerequired.

Some of these other improvements provide for the ability to accomplishso-called sub wavelength patterning of photoresist. By this it is meantthat the techniques employed provide the ability for the electromagneticradiation to pattern features that have dimensions that are smaller thanthe wavelength of the electromagnetic radiation so employed. One suchtechnique is the use of phase shift masks.

Phase shift lithography provides control of the phase of an exposure oflight beam at a target. Adjacent bright areas are formed which arepreferably 180 degrees out of phase with one another. Dark regions areproduced between the bright areas by destructive interference betweenthe radiation phases. One problem with phase shift masks is that theyare typically selected to be compatible with the particular photoresistmaterial being patterned. In order to be effective, the photoresistmaterial being patterned must be closely matched to the phase shift maskbeing used. However, it is desirable to use a wider variety ofphotoresist materials to provide closer spacing and finer definition ofpatterned integrated circuit devices.

What is needed, therefore, is a phase shift mask with variabletransmission properties that allows for the use of differentphotoresists.

SUMMARY

The above and other needs are met by a phase shift mask havingtransmission properties that are dependent at least in part on anintensity of an incident light beam. The phase shift mask has a masksubstrate that is substantially transparent to the incident light beam.A first phase shift layer is disposed on the mask substrate. The firstphase shift layer has a refractive index that is nonlinear with theintensity of the incident light beam. The refractive index of the firstphase shift layer changes with the intensity of the incident light beamon the phase shift mask.

By using a first phase shift layer on the phase shift mask that has arefractive index that is non linear with the intensity of the incidentlight beam, properties of a light beam transmitted through the firstphase shift layer, such as interference patterns in the transmittedlight beam, can be adjusted by adjusting the intensity of the incidentlight beam. Thus, in this manner there is provided an additional tool bywhich the exposure patterns produced by the phase shift mask can beadjusted. In other words, the transmission properties of the phase shiftmask are adjustable with the intensity of the incident light beam.

In another aspect the invention provides a method for making a phaseshift mask. A first phase shift layer is applied to a substantiallytransparent mask substrate. The first phase shift layer is selected fromalkali metal titanyl phosphates. Photoresist is applied to the firstphase shift layer on the mask substrate, and the photoresist isdeveloped to provide a mask pattern. The first phase shift layer isetched according to the mask pattern. The photoresist is stripped fromthe mask substrate to provide the phase shift mask, which exhibits atransmission that is dependent at least in part on an intensity of anincident light beam on the first phase shift layer.

In yet another aspect the invention provides a method for making anintegrated circuit. Photoresist is applied to a layer on a substrate. Anincident light beam is passed through a phase shift mask to produce atransmitted light beam. The phase shift mask includes a mask substratethat is substantially transparent to the incident light beam, and afirst phase shift layer that is disposed on the mask substrate. Thefirst phase shift layer has a refractive index that is nonlinear with anintensity of the incident light beam. The refractive index of the firstphase shift layer changes with the intensity of the incident light beamon the phase shift mask.

The properties of the transmitted light beam as propagated through thefirst phase shift layer are adjusted by adjusting the intensity of theincident light beam. The desired pattern is exposed in the photoresistwith the transmitted light beam, where the adjusted properties of thetransmitted light beam assist in producing the desired pattern. Thephotoresist is developed to provide an etching mask, and the layer onthe substrate is etched to form a structure of the integrated circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages of the invention are apparent by reference to thedetailed description when considered in conjunction with the figures,which are not to scale so as to more clearly show the details, whereinlike reference numbers indicate like elements throughout the severalviews, and wherein:

FIG. 1 is a cross sectional view of a mask substrate containing multiplelayers and photoresist for patterning the phase shift layers,

FIG. 2 is a cross sectional view of the mask substrate containing layersand patterned photoresist for etching patterns in the layers,

FIG. 3 is a cross sectional view of the mask substrate containing layerswherein a first layer is etched according to the pattern provided in thephotoresist,

FIG. 4 is a cross sectional view of the mask substrate containing layerswherein a second layer is etched according to the pattern provided inthe photoresist,

FIG. 5 is a cross sectional view of the mask substrate containing layerswherein a third layer is etched according to the pattern provided in thephotoresist,

FIG. 6 is a cross sectional view of the mask substrate containingpatterned layers according to a first embodiment of the invention,

FIG. 7 is a cross sectional view of the mask substrate containingpatterned layers according to a second embodiment of the invention,

FIG. 7 is a cross sectional view of the mask substrate containingpatterned layers according to a third embodiment of the invention, and

FIG. 8 is a schematic representation of a process using a phase shiftmask made according to the invention to form structures in an integratedcircuit.

DETAILED DESCRIPTION

With reference now to FIGS. 1-8, the invention provides a mask 10 foruse in patterning structures, such as for integrated circuits. The mask10 is patterned as described below to provide areas of adjustable lightbeam transmission through the mask 10. Accordingly, a mask substrate 12is selected from materials that are substantially transparent orsemitransparent to the wavelengths of an incident light beam that is tobe used with the mask 10. Such materials preferably include quartz,glass, calcium fluoride, diamond, diamond-like carbon, fused silica andthe like. A particularly preferred mask substrate 12 material is quartz.The mask substrate 12 preferably causes a phase shift in the incidentlight beam of from about one hundred and seventy degrees to about onehundred and eighty degrees. In other words, a transmitted light beamthat exits the mask substrate 12 has been phase shifted from theorientation of the incident light beam by the mask substrate 12.

A layer 14 of a first phase shift material is applied to the masksubstrate 12. The first phase shift layer 14 is preferably formed of amaterial having a refractive index that is linear with the intensity ofthe incident light beam, and which produces a phase shift in theincident light beam of at least about one hundred and sixty degrees.Such materials include, but are not limited to, magnesium fluoride,titanium dioxide, zinc oxide, aluminum oxide, polyvinylfluoride, siliconoxide, magnesium oxide, molybdenum silicide, molybdenum silicon nitride,carbon, chrome oxide, chrome nitride, silicon nitride, chrome fluoride,chrome oxide fluoride and the like. A particularly preferred materialfor the first phase shift layer 14 is molybdenum silicide.

A second phase shift layer 16 is preferably applied to the first phaseshift layer 14. The second phase shift layer 16 preferably has an indexof refraction that is nonlinear with the intensity of the incident lightbeam, and thus preferably produces a phase shift in the incident lightbeam that is dependent at least in part on the intensity of the incidentlight beam. The second phase shift layer 16 is preferably formed fromalkali metal phosphates such as potassium titanyl phosphate, calciumtitanyl phosphate and rubidium titanyl phosphate. The second phase shiftlayer 16 may also be provided by organic liquid crystal compounds whichare solid at room temperature. Representative examples of suitableorganic liquid crystal compounds include, but are not limited to alkyland alkoxy substituted aryl compounds selected from the following:

N-(p-methoxybenzylidene)-p-butylaniline,

n-pentylphenyl-4-n-methoxybenzoate,

N-(p-methylbenzylidene)-p-butylaniline,

4-n-pentylphenyl-4-n-methoxybenzoate,

4-n-butylphenyl-4-n-pentylbenzoate,

4-n-butylphenyl-4-n-butylbenzoate, and

4-n-pentylphenyl-4-n-butylbenzoate, and the like, which may also includeorganic dyes to enhance the non linearity of the compounds.

Liquid crystal compounds, such as those listed above, may be depositedor formed on the top of the mask substrate 12 or first phase shift layer14 using processes such as chemical vapor deposition. In general,organic nonlinear optical materials are prepared by mixing variouschemical constituents, and allowing them to coalesce on the masksubstrate 12 under specific pressure and temperatures, and under theapplication of a sufficiently strong electric field.

A substantially opaque layer 18 is applied to the second phase shiftlayer 16. The opaque layer 18 is preferably formed from metals and metaloxides and nitrides including but not limited to chrome, titaniumdioxide, zinc oxide, aluminum oxide, silicon oxide, magnesium oxide,chrome oxide, chrome nitride, silicon nitride and the like. Aparticularly preferred material for layer 18 is chrome or chrome oxide.

Each of the layers 14, 16 and 18 may be applied to or deposited on themask substrate 12 by a variety of microelectronic processing techniques,including but not limited to chemical vapor deposition, sputtering, spincoating, and the like. The thickness of each layer 14, 16 and 18typically ranges from about five hundred angstroms to about one thousandangstroms or more.

Photoresist 20 is also applied to the mask 10. A desired pattern isformed in the photoresist, as depicted in FIG. 2, which shows trenches22 and islands 24. It is appreciated the pattern as depicted in FIG. 2is representational only, and that in actual practice of the invention,a wide variety of shapes may be patterned in the photoresist 20.

The photoresist 20 provides a pattern for forming etched features in thelayers 14, 16, and 18, as illustrated with reference to FIGS. 3-7. It ispreferred to us etchants that are relatively highly selective for thephase shift layer being etched. In this manner, different layers may beetched in different patterns. The mask substrate 12 may also beselectively etched in different portions of the pattern.

Although only a single etched patterned is depicted in the figures anddescribed in the example that follows, it is appreciated that byremoving and reapplying photoresist layers with differing patterns, awide variety of different patterns of the various layers 14, 16, and 18can be formed, where different ones and combinations of the variouslayers are present in different portions of the mask 10. This isadditionally facilitated by patterning the various layers 14, 16, and 18prior to depositing a subsequent layer 14 or 18.

It is also appreciated that the number of such layers 14, 16, and 18 isalso representational, and that different numbers of these and otherlayers may be used, and in different combinations of layers, to providevery complicated and specifically designed stack structures. Further,the layers 14, 16, and 18 can be applied in an order that is differentfrom that as described herein. Thus, the specific embodiments describedherein are extremely basic in nature, and are only given for the purposeof clarity in the description, and not by way of limitation.

To continue with the basic example, in the first etching step the layer18 is etched (FIG. 3) to provide features corresponding to trenches 22and islands 24 (FIG. 2). Next, the second phase shift layer 16 is etchedas shown in FIG. 4. Finally, the first phase shift layer 14 is etched toprovide the etched features illustrated in FIG. 5. After etching thelayers 14, 16 and 18, the photoresist 20 is preferably removed and thephase shift mask 10 is cleaned, as depicted in FIG. 6.

FIG. 7 depicts an alternate embodiment of the phase shift mask 10, wheredifferent photoresist layers 20 have been employed at different times toproduce a more complex phase shift mask 10. FIG. 8 depicts an embodimentof the phase shift mask 10 without the first linear phase shift layer14, where the second nonlinear phase shift layer 16 is used inconjunction with the opaque layer 18.

The phase shift mask 10 is used to pattern a wide variety ofphotoresists 26, such as are applied to an integrated circuit substrate28, as depicted in FIG. 9. An advantage of the invention is that a widervariety of resist materials may be used for layer 26, since the phaseshift layers 14 and 16 of the phase shift mask 10 may be selected frommaterials having refractive indices that are both linear and nonlinearwith the intensity of the incident light beam 30.

For example, during a patterning process using the phase shift mask 10,a source such as an excimer laser 36, using a krypton fluoride, argonfluoride, or fluorine gas source for example, provides an incident lightbeam 30 that is directed through a lens 32 and the phase shift mask 10to produce a transmitted light beam 34 that is focused on thephotoresist layer 26 of the integrated circuit substrate 28. Afterexposing the photoresist 26, conventional photolithography processes arepreferably used to create the desired structures in the substrate 28.

The foregoing description of preferred embodiments for this inventionhave been presented for purposes of illustration and description. Theyare not intended to be exhaustive or to limit the invention to theprecise form disclosed. Obvious modifications or variations are possiblein light beam of the above teachings. The embodiments are chosen anddescribed in an effort to provide the best illustrations of theprinciples of the invention and its practical application, and tothereby enable one of ordinary skill in the art to utilize the inventionin various embodiments and with various modifications as is suited tothe particular use contemplated. All such modifications and variationsare within the scope of the invention as determined by the appendedclaims when interpreted in accordance with the breadth to which they arefairly, legally, and equitably entitled.

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 7. An integrated circuit having a structure with featuresizes smaller than a wavelength of the incident light beam, thestructure patterned with a phase shift mask having transmissionproperties that are dependent at least in part on an intensity of anincident light beam, the phase shift mask comprising: a mask substratethat is substantially transparent to the incident light beam, and apatterned first phase shift layer disposed on the mask substrate, thefirst phase shift layer having a refractive index that is nonlinear withthe intensity of the incident light beam, wherein the refractive indexof the first phase shift layer changes with the intensity of theincident light beam on the phase shift mask.
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 20. An integrated circuit formed with the method of:applying photoresist to a layer on a substrate, passing an incidentlight beam through a phase shift mask to produce a transmitted lightbeam, where the phase shift mask includes a mask substrate that issubstantially transparent to the incident light beam, and a first phaseshift layer disposed on the mask substrate, the first phase shift layerhaving a refractive index that is nonlinear with an intensity of theincident light beam, wherein the refractive index of the first phaseshift layer changes with the intensity of the incident light beam on thephase shift mask, adjusting properties of the transmitted light beam aspropagated through the first phase shift layer by adjusting theintensity of the incident light beam, exposing the desired pattern inthe photoresist with the transmitted light beam, where the adjustedproperties of the transmitted light beam assist in producing the desiredpattern, developing the photoresist to provide an etching mask, andetching the layer on the substrate to form a structure of the integratedcircuit, where the structure of the integrated circuit has feature sizessmaller than a wavelength of the incident light beam.